Thermosetting divinyl polyester resin is often used in molding compounds, in casting or in laminate matrix material. Unfortunately, divinyl polyester resins are inherently brittle. Therefore, whenever divinyl polyester resins are used as a matrix, it must be toughened. Normal industry practice is to add a flexibilizer to the resin composition. However, this often results in a significant reduction in the mechanical properties which is unacceptable for many product applications. It is desirable to toughen divinyl polyester resin compositions without significant adverse effects on other important properties, such as cure rate, maturation and strength.